Silicon nitride sintered products are ceramics which are extremely hard and exhibit excellent wear resistance as well as strength and toughness. Therefore, silicon nitride sintered products have been used in cutting tools and wear-resistant components.
In JP-A-1-157466 (1989) (the term "JP-A" as used herein means an "unexamined published Japanese patent application") which corresponds to U.S. Pat. No. 4,880,756 a silicon nitride sintered product in which Mg, Zr, Al or the like is used as a sintering aid is proposed as a product having excellent wear resistance. This prior art teaching is based on the finding that Al and the like contribute to product densification by forming, together with Si , N and O, a liquid phase in the interstices between Si.sub.3 N.sub.4 grains during firing, and also bond Si.sub.3 N.sub.4 grains by glassification during cooling to avoid debonding of Si.sub.3 N.sub.4 grains during abrasion, thereby enhancing toughness. In some cases, a wear-resistant film may be formed on the surface of the base material of a cutting tool, giving it excellent strength and toughness so that the wear resistance is improved.
However, in these cutting tools, despite the wear-resistant components and the reciprocating components, still further alleviation of wear is required to prolong tool life, since the techniques described in JP-A-1-157466 do not satisfy such requirements. Although it was possible to obtain the sintered products which exhibit excellent defect resistance by improving the strength and toughness, there was previously no specific means for improving the wear resistance, since the relationship between wear resistance and mechanical properties was not known.